There are a number of practical applications for remote detection and, if possible, imaging of gaseous species present in a low concentration. These include remote detection of leaks of inflammable or poisonous materials and remote detection of explosives. At present, it is difficult to detect and particularly to image materials remotely in sufficiently low concentrations, because the available techniques are not sufficiently powerful to detect materials in low concentrations reliably or sufficiently able to discriminate relevant species.
One particularly promising technique is back-scatter absorption gas imaging (BAGI). This technique involves providing a source of light tuned to a wavelength where the target species has an absorption band, and a detector for detecting light scattered from a target area. The presence of gas will occlude an image of a scene from the target area captured where there is no gas absorption (for example, at another wavelength where there is no absorption from the target species).
It is desirable for the linewidth of the light source to be equal to or less than the width of the absorption band. For short chain hydrocarbon molecules, absorption bands of interest lie in the 2-4 micron range. For these parameter constraints, a particularly suitable light source is an optical parametric oscillator (OPO) using a nonlinear crystal such as periodically poled lithium niobate (PPLN). An OPO is a complex optical source which comprises a pump laser and a nonlinear crystal. The nonlinear crystal converts the pump light into two lower frequency (and hence longer wavelength) waves by virtue of a second order nonlinear optical interaction. The sum of the frequency of these two output waves is equal to the frequency of the pump input. The lower frequency (and longer wavelength) output is termed the idler, and the higher frequency (and shorter wavelength) output is termed the signal.
The use of BAGI techniques using OPO light sources has been extensively studied at Sandia National Laboratories (SNL). Representative papers from this research group include “Backscatter Absorption Gas Imaging—a New Technique for Gas Visualization” by T. G. McRae and T. J. Kulp, Applied Optics, 1993, 32(21) pp. 4037-4050; “Active infrared imagers visualize gas leaks” by T. J. Kulp and T. McRae, Laser Focus World, 1996, 32(6) p. 211; and “Demonstration of differential backscatter absorption gas imaging” by P. E. Powers et al, Applied Optics, 200, 39(9), pp. 1440-48. Systems using both continuous wave and pulsed OPOs are described, and imaging systems are described including focal-plane array cameras and rastering scanners. However, these systems are generally expensive and immobile, and not well adapted to real world applications outside a laboratory environment.
A development on this approach is described in WO 2006/061567 A1. This discloses a BAGI system using an OPO in which the pump wave laser source and the nonlinear medium are provided in the same optical cavity. This approach allows for more efficient use of pump laser power, and in combination with use of Q-switching, allows for use in a rapidly pulsed mode which can be used effectively with raster scanning to construct an image of a scene. This makes it possible to produce a less expensive and more mobile device capable of IR imaging using BAGI techniques.
While these techniques are effective to image the presence or absence of classes of material, such as short chain hydrocarbons, they lack the resolution to allow specific materials of interest to be distinguished from a more general class. This is because use of OPOs of this type only allows access to the medium wavelength infrared (MWIR) region, typically defined as extending from 3-8 μm and shorter wavelengths—for example, the working range of a PPLN OPO is typically from 2-4 μm. This MWIR region contains absorption bands which are effective to allow a specific class of material (such a ketone, an unsaturated hydrocarbon or a saturated hydrocarbon) to be recognised, but not to allow one material within that class to be distinguished from another. Recognition of individual molecular species typically requires a spectrum over a broader spectral region. Multiple spectral bands, including bands in the long wavelength infrared (LWIR), typically defined as extending from 8-15 μm, can then be used and matched with known or calculated spectra to determine the presence or absence of a particular species. The “fingerprint region” for infrared spectroscopy lies largely in the LWIR—the fingerprint region is normally taken as extending between 500 and 1500 cm−1, or 6.67-20 μm. Spectral lines in the fingerprint region generally include relatively sharp lines which result from bending vibrations specific to the geometry of an individual molecule—these spectral lines distinguish different members of a class from each other and can thus be used to identify individual molecular species. Existing BAGI techniques cannot however work effectively in most of the signature region, as known technologies do not function effectively beyond the MWIR region.